What is the water-cement ratio (w/c) required to complete the reactions in hydration of cement?
Correct Answer: C. 0.35 to 0.45
📚 Detailed Explanation: W/C Required for Complete Cement Hydration
Cement hydration involves two distinct types of water binding: (1) non-evaporable (chemically combined) water that becomes part of the hydration products, and (2) gel water adsorbed in the C-S-H gel. Both types must be available for complete hydration.
Why C (0.35 to 0.45) is correct:
• Non-evaporable water (for chemical reactions) = ~0.23 × mass of cement (w/c = 0.23 minimum for chemical reactions alone).
• Gel water (adsorbed in C-S-H gel structure) = ~0.19 × mass of cement.
• Total for complete hydration = 0.23 + 0.19 = 0.42 → range is approximately 0.35–0.45.
Option B (0.20–0.30) is only enough for partial hydration. Option D (0.50–0.65) is a practical workability range, not the minimum for hydration.
• Non-evaporable water (for chemical reactions) = ~0.23 × mass of cement (w/c = 0.23 minimum for chemical reactions alone).
• Gel water (adsorbed in C-S-H gel structure) = ~0.19 × mass of cement.
• Total for complete hydration = 0.23 + 0.19 = 0.42 → range is approximately 0.35–0.45.
Option B (0.20–0.30) is only enough for partial hydration. Option D (0.50–0.65) is a practical workability range, not the minimum for hydration.
Water Requirements in Cement Hydration
| Type of Water | W/C Required | Purpose |
|---|---|---|
| Chemical (non-evaporable) | ~0.23 | Forms hydration products (C-S-H) |
| Gel water (adsorbed) | ~0.19 | Fills gel pores in C-S-H |
| Total for complete hydration | ~0.35–0.45 | All hydration reactions |
| Practical minimum (workability) | 0.40 | Workable concrete |
Key Concepts for Students
- At w/c < 0.35, hydration is incomplete — some cement never hydrates, leaving unreacted clinker particles.
- At w/c > 0.45, all hydration reactions complete; the excess water creates capillary pores that reduce strength.
- 0.38 is often cited as the practical w/c for complete hydration under normal curing conditions.